1. Field of the Invention
This invention relates to a semiconductor device, and more particularly to a transistor structure having a ring-shaped emitter region formed in a base region.
2. Description of the Prior Art
There has recently been developed the so-called ring emitter transistor which has a ring-shaped emitter region. FIG. 1 shows in perspective one part of such a transistor. In FIG. 1, reference numeral 1 indicates a silicon semiconductor substrate; 2 designates an epitaxial layer formed on the substrate, which layer is a collector region; and 3 identifies a base region formed on the collector region. The base region 3 has formed therein a plurality of base electrode forming regions 4 which have a higher impurity concentration than the base region 3 and which are interconnected through aluminum (Al) wiring layers for wiring base electrodes, though not shown. In the base region 3, a plurality of ring-shaped emitter regions 5 are formed and an emitter electrode forming region 6 is formed substantially centrally of each ring-shaped emitter region 5 and, further, the emitter region 5 and the emitter electrode forming region 6 are interconnected through an impurity diffused region 7 having a relatively high resistance and serving as a ballasting resistor. The emitter electrode forming regions 6 are interconnected through aluminum wiring layers for wiring emitter electrodes.
The transistor of such a construction is fabricated as an integrated circuit with a plurality of parallel connected ring emitter transistor units. Each of the transistor units are electrically equivalent to one another and have substantially the same electrical properties. Since the transistor units can be actuated to perform uniform operation through the ballasting resistors individually provided for the units, it is possible to obtain a large power output from the transistor while retaining the high-speed operability of a small-power unit, and to retain excellent power handling capability, too. If the value of the ballasting resistor provided for each ring emitter transistor unit as described above is small, the negative feedback action of the ballasting resistor decreases to lessen its effect. With a large resistance value, however, there occurs such an incompatible phenomenon that the power handling capability increases, whereas a maximum current, for example, a maximum collector current providing a current amplification factor h.sub.FE = 10, decreases. Accordingly, in the actual transistor design, it is difficult to set the value of the ballasting resistor.